Re: RE: [geonet] Terrain Grid Problem

[edcunion <edcun...@yahoo.com>]

Hi Caitlin    I'm not at the computer but yes, believe the grid stats can be recalculated under grid properties and it might display ok after saving the stats changes without having to export to an xyz.   The 167 km radius follows the old Hayford bowie  outer TC zone and the Bullard slab curvature correction if used also follows this radii convention. Believe the Bullard TC does too. Believe Donald plouff @ Usgs also followed this with his old program but would have to check.   Agree with Rafael sticking to a 20 km regional often works well and is real quick to generate the TC.  it's easy in geosoft to check the TC numbers from say a 20 km radius against a larger TC radius and to create a difference TC database column and grid to see exactly how the two TC differ and affect the complete bouguer. Two complete bouguer data columns and grids can then be quickly made for a subsequent bouguer difference comparison   Ed       Sent from my T-Mobile 4G LTE device     ------ Original message------ From: author.nameemail Date: Tue, 12/29/2015 4:43 PM To: Geosoft Oasis montaj and Target; Subject:RE: [geonet] Terrain Grid Problem   Dear Ed and Rafael,   Thank you for your emails.  They are all very helpful.    My grids have the following characteristics: 90m SRTM 167km from every stations 10m local 2000m out from every station The final terrain correction grid in this case would be 83 km x 93 km so I am under the 400km that you noted.  The optimization was set to faster.   I checked the grid statistics on the terrain correction grid and there are no values reported.  I exported the grid to an XYZ file and then regridded it and it gridded up fine.  Now I just need to test if the grid values are valid.  It appears to me that the terrain grid MAY not have closed itself properly since I shut the machine down but the grid was actually ok.  Maybe it was the statistics it was working on.  Anyway, it looks like I have something to work with now.   Thanks again for all of the good tips and suggestions.   Caitlin     From: edcunion <edcun...@yahoo.com> [mailto:geonet@lyris.geosoft.com] Sent: Tuesday, December 29, 2015 9:01 AM To: Geosoft Oasis montaj and Target Subject: Re: [geonet] Terrain Grid Problem   Hi,   not sure what has happened to take 3 days to create a large terrain grid in your example. Perhaps the grid cell sizes of your two input DEM and output terrain correction grids was set very fine, or the terrain correction (TC) optimization was not set to faster? Did you mask and minimize your regional input DEM to be a window less than <=400 km on a side?   The gravity module has been used here to create TC grids using regional 90 m cells from SRTM data that go out to ~ 150 -200  km from a station and local DEM 25 m cells that go out to 1000 m and these took several minutes but quite a bit less than one hour using the Geosoft gravity module (on a 4 year old quad processor laptop). The TC optimization setting was set to "faster" in this example. The final TC grid was a few mb in size, not into the hundreds of mb. What grid cell sizes were used for your local and regional DEM grids that created the TC grid?   When creating the TC grid I'd first double check the input grid cell sizes on both your local correction DEM grid (local GPS elevations from the ground gravity survey?) and your regional correction DEM (SRTM or other regional?) grid. The "grid properties" tool can be used to look at both grid cell sizes from your two DEM grids being used to create the TC grid, and used to look at the total number of cells in your input grids. It can also be used to look at the same properties of your large 250 mb TC grid.   I'm guessing if the two input DEM grid cell sizes are set to reasonable numbers and if the regional DEM is a rectangle that is less than 400 km on a side, and TC optimization is set to faster, a new TC grid will be created in several minutes that will be quite a bit less than 250 mb in size and it should then be viewable. Before running the TC though the "scan XY" radio button at the bottom of the TC menu can be used to check the extent of your survey limits before the TC is run. You can also specify numbers into your outer and inner TC correction distances for more control in the TC menu.   If nothing shows on your original large 250 mb output TC grid and the grid properties check indeed verifies there are TC numbers in the grid, you can try changing the color stretch to see if anything shows up after a linear, normal or equal area stretch is used, using either the default Geosoft color table or the Geosoft elevation color table.   If the above does not help perhaps Geosoft could advise after they'd look at your input grids and data.   Cheers,   Ed   Ed Cunion Red Rocks Geophysical 13224 W Utah Circle Lakewood CO 80228 USA cell 001-720-300-3641 fax/phone 001-720-389-5224 Skype ed.cunion         On Monday, December 28, 2015 5:51 PM, author.nameemail <geonet@lyris.geosoft.com> wrote:   Hello,   I created a very large terrain correction grid using the gravity module of Geosoft Oasis Montaj.  It took over 3 days to create the terrain correction grid.  Once it was done, it locked up my computer for another day and I finally had to shut it off.  The TC grid is about 250 megs but if I open it in Oasis Montaj, it shows a blank screen.  Is there any possibility that this grid may still be usable?  If so, what do I need to do to make it show up on the screen.    Secondly, every time I create large terrain correction grids, my computer locks up after it completes.  What can I do about this?  Anything?  I often end up having to shut my computer down ungracefully and I do not like doing that.  This is not an old machine, it is actually pretty new and powerful.   Thank you for your help.   Caitlin Bernier-GISP Magee Geophysical Services LLC caitlin_bern...@gravityandmag.com   --- Forum archives can be accessed here: http://lyris.geosoft.com/read/?fourm=geonet   You are currently subscribed to geonet as: edcun...@yahoo.com. 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